Machine having a converter controlled drive

ABSTRACT

A with a converter controlled drive includes a rotor, a drive having a variable rotational frequency a working machine and a frequency converter. The drive and the working machine are connected to one another such that a torque is transmitted. The frequency converter is electrically connected to the drive and converts input frequencies to output frequencies. The output frequencies are grouped into a plurality of concentration ranges based upon a rotational speed of the machine. Each concentration range defines a blocking range. The machine has an operating rotational speed range which lies outside the blocking ranges.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/636,179, filed on Nov. 27, 2012, and claims the benefit thereof under35 U.S.C. §120. The U.S. patent application Ser. No. 13/636,179US is theNational Stage of International Application No. PCT/EP2011/054366 filedMar. 22, 2011, and claims the benefit thereof. The InternationalApplication claims the benefits of German Patent Application No. 10 2010012 268.8 DE filed Mar. 22, 2010, and International Application No.PCT/EP2011/054225 filed Mar. 21, 2011. All of the applications areincorporated by reference herein in their entirety.

FIELD OF INVENTION

The invention relates to rotary machines, particularly to a machinehaving a converter-controlled drive with a variable rotationalfrequency.

BACKGROUND OF INVENTION

Such machines have at least one rotor and can also comprise a pluralityof rotors which can also be components of an intermediately arrangedtransmission. If there is a plurality of rotors, the invention can beapplied to each individual. When there is a single rotor, the driverotor is permanently connected to the working machine rotor along acommon rotational axis to form a shaft line.

An exemplary field of application of the invention is the power range ofat least 1MW absorption power which differs basically in terms of thedimensions, the selection of materials and the use of significantlysmaller assemblies.

As a result of the rectifying and, in the next step, the inversionwithin the converter to the output frequency or the desired workingfrequency, not only the working frequency but also harmonic andinter-harmonic frequency components are generated in the electrical feedsignal of the motor.

If the Fourier analysis (rapid Fourier transmission) is also used, interalia, to determine frequency components which are not integral multiplesof the frequency of the feed signal, these components are referred to asinter-harmonics.

These harmonic and inter-harmonic frequencies in the electrical feedsignal of the motor are applied to the mechanical system in the air gapof the motor as exciting torsional moments.

A Campbell diagram permits the running performance of a machine in arotational speed range to be assessed through this synopsis of therotational speed, exciter frequencies and natural frequencies. The Xaxis of the Campbell diagram or the abscissa represents rotational speedof the rotor of the machine under consideration. If an oscillationprofile which is dependent on the rotational speed, for example atorsional oscillation of a rotor shaft, is transformed from the timedomain to the frequency domain by means of Fourier transformation, theseare represented in the Campbell diagram as a rising and a fallingstraight line plotted against the X axis, wherein the latter representthe rotational speed of the rotor. Orders (O1, O2, . . . ) of theFourier transformation are then reflected in these straight lines whichappear as center point beams and whose gradient is proportional to therespective ordinal number. The frequency f of the natural frequency ofthe rotor or the rotating part which is subjected to consideration isrepresented on the ordinate. The natural frequencies are represented asa tolerance band whose respective width arises as a result of theinaccuracy of the model formation and, if appropriate, other variants.As a result, the natural torsional frequency relates, unless statedotherwise, to the described tolerance band in all cases. The bandwidthof the tolerance band is already obtained from irregularities of thegeometry due to unavoidable fabrication tolerances. A tolerance band is,for example, assumed to be wide here such that a calculation directlyincludes various embodiments of the machine, with the result that thesevariants are also covered by the dimensions. Accordingly, the toleranceband can, for example, have a certain lack of precision.

In addition, harmonic exciter frequencies are represented, which arerepresented as straight lines parallel to the abscissa if they areindependent of the rotational speed. If the exciter frequency varieswith the rotational speed, said frequency is represented as a rising orfalling straight line through the origin. If the rotational speed of themachine is in a range in which the exciter frequency profiles intersectthe tolerance band of natural frequencies, increased oscillationaltitudes are to be expected.

Inter-harmonic exciter frequencies occur as V-shaped, symmetrical beamsfor output frequencies F1, F2, F3, . . . ; Fn in the Campbell diagram.Wherein F1, . . . Fi, . . . Fn are grouped into concentration ranges G1,. . . , Gi, . . . Gz, wherein Fi which are close to one another andwhich together form a common output point are combined in Gi.

The upper and lower limits of the concentration range G1, . . . , Gi, .. . Gz are defined by the intersection point of the lowest naturaltorsional frequency of the rotor with the two straight lines of the beampair of the inter-harmonics of the first order of the respectiveconcentration range G1, . . . , Gi, . . . Gz. The intersection point inthe case of inter-harmonics always denotes the coordinates with thehighest frequency with respect to the range of the tolerance band whichis intersected by the inter-harmonics. Insofar as an excitation of thesecond and/or third natural torsional frequency is mechanicallypossible, this is to be taken into account in the same way (mutatismutandis) as described above for the first natural torsional frequency.

If the harmonic and inter-harmonic exciter frequencies are representedtogether with the natural torsional frequencies of the mechanical systemin a Campbell diagram (exciter or natural frequencies plotted againstthe rotational speed of the motor), it is seen that in the operationalrange of conventionally configured motors, intersection points of thenatural torsional frequency which can be excited by the motor (usuallythe first natural torsional frequency) with the inter-harmonic exciterfrequencies occur. A steady-state operation of the mechanical system atone of these intersection points of inter-harmonic excitation andnatural torsional frequency leads to a state of resonance with hightorsional oscillation amplitudes and therefore to high dynamic torsionalstresses in the torque-transmitting line components. The consequenceswhich possibly result from this, for example fatigue damage to the loadof the line components, should be avoided.

Drives with converter-controlled electric motors have as a rule afrequency converter and an electrical synchronous motor or asynchronousmotor. While the input frequency into the converter is embodied as apure sinusoidal oscillation on the basis of the virtually perfectrotational movement of the energy generation assemblies which feed thepower system frequency, the spectrum of the frequency analysis showsthat the output from the converter has, in addition to the set pointfrequency, other frequencies which can lead to excitations of torsionaloscillations. Such undesired secondary frequencies, which have beenvirtually impossible to avoid hitherto, are also referred to as harmonicor inter-harmonic exciter frequencies. The inter-harmonic exciterfrequencies within the customary operating rotational speed range of themotor usually give rise to excitation of torsional oscillations of theentire drive, for example driven compressor trains or other turbo sets.

Insofar as there is no intermediate transmission in the mechanicaltrain, the additional loading, caused by the excited torsionaloscillation, can occur largely unnoticed. However, the undesired dynamicadditional loading in the mechanical line components give rise to aconsiderably shortened service life owing to fatigue of components.

If a transmission is a component of the machine, within the transmissiontoothed engagement occurs to form a coupling between torsionaloscillations and radial oscillations. As a result, the torsionaloscillations in the transmission also have the effect of shortening theservice life. In addition, undesired large radial oscillations andundesired increased noise emission (rattling of the transmission) occur.

The problem of undesired torsional oscillations can be frequentlydetected only by means of a dynamic measurement of the torsional moment.Such a measurement is usually not used for continuously monitoring aturbo line, and would only identify torsional resonances which arepresent but would not avoid the cause of their generation.

SUMMARY OF INVENTION

It is desirable to improve the smooth running of machines withconverter-controlled drives and of avoiding the possible consequences oflarge oscillations, for example fatigue damage, as a result.

The above may be achieved by the features of the independent claim(s).

An operating rotational speed range specifies here the range of therotational speed of a rotor which is used as the basis for theconfiguration of the machine and in which the rotational speed islocated during at least 90% of the assumed operating period.

One embodiment of the invention provides that the converter-controlleddrive has a number of pole pairs which, in conjunction with apredetermined motor feed frequency operating range of the converter,defines a motor feed frequency and/or an operating rotational speedrange, lying outside the concentration ranges G1 to Gz.

An exemplary field of application of the invention includes machineswhich are embodied as a turbo set, in particular machines which have atleast one turbo compressor. The currently customary selection of thedrive has the effect that the concentration ranges G1, . . . Gz giverise to intersection points with the natural torsional frequency in thedriving range and therefore large torsional loadings of the linecomponents. Insofar as the torque is transmitted from aconverter-controlled drive to the working machine, it is possible, givenknowledge of the inter-harmonic torsional excitations, to avoidtorsional resonance states in the operating rotational speed range ofthe drive through a targeted selection of the number (PPZ) of pole pairsof the drive. In this way, the motor avoids the natural inter-harmonicexciter frequency of the converter by shifting the operationalrotational speed range to a lower or higher rotational speed range. Whena transmission is used, further resonance-free operating rotationalspeed ranges which arise can be used to selectively adjust thetransmission ratio. For example it may be expedient here if thetransmission adjusts the rotational speed of the converter-controlleddrive to a relatively high rotational speed at the working machine. Inparticular, a number of pole pairs of >2 of the converter-controlleddrive in combination with a transmission, which transmits to arelatively high rotational speed, for the working machine can implementthe inventions if at least one concentration range lies in the region ofthe input frequency into the converter, which is relatively frequentlythe case. In this way, the motor avoids the inter-harmonic exciterfrequency of the converter in the direction of a relatively lowrotational speed, and the transmission adjusts this relatively lowrotational speed to the desired rotational speed of the working machineor into a corresponding operating rotational speed range.

Insofar as the second and/or third natural torsional frequency ismechanically possible, it should be taken into account in the same way(mutatis mutandis) as described above for the first natural torsionalfrequency.

As a result, a specific exemplary embodiment is described, wherein thisserves merely to illustrate the invention and other possibilities forimplementing the invention are conceivable to a person skilled in theart, in particular by means of any desired combination of the featuresdefined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified illustration of a machine according to theinvention,

FIG. 2 shows a Campbell diagram, characteristic of the converterillustrated by way of example in FIG. 1, and

FIG. 3 shows a detail of the Campbell diagram in FIG. 2 and theimplementation possibilities with respect to the operating rotationalspeed range of the machine.

DETAILED DESCRIPTION OF INVENTION

FIG. 1 is a schematic view of a machine M according to the invention.Significant components of the machine M according to the invention arethe (frequency) converter VFG, a converter-controlled drive VFD with avariable rotational speed n₁, and a working machine WM, which isembodied here as a compressor CO. The exemplary embodiment furthermorealso comprises a transmission TR, which converts the rotational speedgenerated by the drive VFD at a first shaft SH1 to a second rotationalspeed n2 at a second shaft SH2, which drives the compressor CO. Thecompressor CO conveys a mass flow M1 from a first pressure P1 to ahigher second pressure P2. The converter VFG generates, from the powersystem frequency of, for example, 50 Hz (input frequency fE), an outputfrequency f0, with which the drive is fed. The drive VFD rotates as afunction of the output f0 from the converter VFG and as a function ofthe number of pole pairs (NPP=number of pole pairs) with the rotationalspeed n1. The rotational speed n1 corresponds here to the quotient fromthe output frequency f0 from the converter VFG and the number of polepairs NPP. The arrangement composed of the drive VFD, the first shaftSH1, the transmission TR, the second shaft SH2 and the working machineWM has, with respect to the shafts, natural torsional frequencies FT1,FT2, FTi, with the result that high oscillation amplitudes can occurduring operation in the vicinity of these frequencies. Furthermore, as aresult of the torsional bending oscillation coupling, in particular inintermediate transmissions, radial oscillations can also be excited bytorsional oscillations. The converter VFG is a currentintermediate-circuit converter (LCI). Alternatively, the converter canalso be a voltage converter.

As already explained at the beginning, the output frequency f0 which isgenerated by the converter VFG can be represented as a pure sinusoidaloscillation with a set point frequency on which further components ofharmonic and inter-harmonic oscillations with a different frequency aresuperimposed.

FIG. 2 shows in this respect a schematic illustration of the Campbelldiagram, which represents these so-called inter-harmonic exciterfrequencies of the converter output. These exciter frequencies aregenerated in the converter and transmit as a torque fluctuation in themotor, via the air gap between the stator and rotor, to the mechanicalsystem.

The Campbell diagram is used in all fields of oscillation technology,for example for representing oscillations of rotors and blades. Thediagram is suitable for assessing the oscillation adjustment, forexample in the entire operating rotational speed range, and foridentifying possible resonance states.

Natural frequencies can also be represented as a frequency band whosewidth results from the variance of calculation models. In addition tothe harmonic exciter frequencies, there are also spectral components forthis type of application case, which components are referred to asinter-harmonic exciter frequencies. In the Campbell diagram in FIGS. 2and 3, the inter-harmonic exciter frequencies FIH1, FIH2, FIH3 are eachrepresented as straight lines which each have a symmetrical partner,which straight line partner has the same gradient value with a differentsign. On the abscissa of the Campbell diagram, the straight lines, whichindicate the dependence of the inter-harmonic exciter frequencies withrespect to the drive rotational speed, intersect. In addition to theharmonic exciter frequencies, which are represented in the Campbelldiagram in FIG. 2 as a group of straight lines through the zero point ofthe diagram, FIG. 2 shows by way of example three straight line groupswhich indicate inter-harmonic exciter frequencies as function of thedrive rotational speed, said frequencies each having a starting point onthe abscissa.

In FIG. 3, the case of various numbers of pole pairs for the drive(NPP=1, 2, 3) is shown. In the upper diagram, typical natural torsionalfrequencies of possible turbo compressor trains with a certain widthcorresponding to the variance are shown as a horizontal line. Theharmonic and inter-harmonic frequency profiles corresponding to thestraight line through the origin or through the first concentrationpoint CP are generated by the converter. The intersection points of thestraight lines (shown here by way of example only as an intersectionpoint of the upper limit of the natural torsional frequency range) withthe natural torsional frequency band RL of the compressor train resultin rotational frequencies n1, . . . ,ni, which are grouped around theconcentration point CP. The range FA which is defined by the outerelements of the grouping is not part of the aimed-resonance-freeoperating rotational speed range of the range OR, entered as an example,of the machine M. The operating rotational speed range OR which is shownis arranged here, for example, between the spanned blocking ranges ofthe harmonic exciter frequencies HEF at a low rotational speed and theblocking range FA which is spanned by the inter-harmonic exciterfrequencies iHEF. The operating rotational speed range OR is shown hereby way of example and can, of course, be positioned in all the possibleresonance-free rotational speed ranges, therefore also above theconcentration range shown here. According to the invention, the outputfrequency FO of the converter can be selected in combination with thenumber of pole pairs NPP of the drive VFD in such a way that nosignificant torsional excitations due to inter-harmonic exciterfrequencies occur within the desired operating rotational speed range.The position of the inter-harmonic exciter frequencies is dependent onthe characteristic and the input frequency (power system frequency) FEinto the converter VFG. With the number of pole pairs NPP, the positionof the operating rotational speed range OR with respect to the blockingrange FA which is spanned by the inter-harmonic exciter frequencies canbe determined. As illustrated in the example diagram c), a number ofpole pairs NPP=2 can shift the blocking range about a concentrationpoint CP about 1500 RPM instead of about 3000 RPM given a number of polepairs NPP=1. Correspondingly, the width of the concentration range CP ofthe blocking range FA is reduced to a third at 1000 RPM, given a numberof pole pairs of NPP=3.

What is claimed is:
 1. A machine comprising: a rotor, aconverter-controlled drive with a variable rotational frequency, aworking machine, wherein the drive and the working machine are connectedto one another such that a torque is transmitted, a frequency converterwhich is electrically connected to the drive and converts inputfrequencies to output frequencies, wherein the output frequencies aregrouped into a plurality of concentration ranges based upon a rotationalspeed of the machine, wherein each concentration range defines ablocking range, and wherein the machine has an operating rotationalspeed range which lies outside the blocking ranges.
 2. The machine asclaimed in claim 1, wherein the frequency converter is embodied suchthat in a Campbell diagram, relating to the machine, intersection pointsresult from the natural torsional frequency of the rotor and V-shapedsymmetrical straight lines of the inter-harmonic exciter frequency forthe output frequencies.
 3. The machine as claimed in claim 2, whereineach concentration range comprises output frequencies which are close toone another and which respectively have a common output point on theabscissa of the Campbell diagram.
 4. The machine as claimed in claim 3,wherein upper and lower limits of each concentration range is defined byan intersection point of a lowest and/or second lowest and/or thirdlowest natural torsional frequency of the rotor and two straight linesof a pair of the inter-harmonics of the first order of the concentrationrange.
 5. The machine as claimed in claim 1, wherein the drive comprisesa plurality of pole pairs which, together with a motor feeding frequencyrange, define the operating rotational speed range which lies outsidethe blocking ranges.
 6. The machine as claimed in claim 1, wherein atransmission for transmitting the rotational speed of the drive to arotational speed of the working machine acts between the drive and theworking machine.
 7. The machine as claimed in claim 6, wherein therotational speed of the working machine is higher than the rotationalspeed of the drive.
 8. The machine as claimed in claim 1, wherein thefrequency converter is a current intermediate-circuit converter.
 9. Themachine as claimed in claim 1, wherein the frequency converter is avoltage converter.
 10. The machine as claimed in claim 1, wherein thedrive is a synchronous motor.
 11. The machine as claimed in claim 1,wherein the drive is an asynchronous motor.
 12. The machine as claimedin claim 1, wherein a maximum absorption capacity of the drive is atleast one megawatt.
 13. The machine as claimed in claim 1, wherein theworking machine is a turbo-machine.
 14. The machine as claimed in claim1, wherein the working machine is a turbo compressor.
 15. A method ofoperating a machine with a rotor, comprising: providing aconverter-controlled drive with a variable rotational frequency and aworking machine, connecting the drive and the working machine such thata torque is transmitted, connecting electrically a frequency converterto the drive, wherein the converter converts input frequencies to outputfrequencies, wherein the output frequencies are grouped into a pluralityof concentration ranges based upon a rotational speed of the machine,wherein each concentration range defines a blocking range, and whereinthe machine has an operating rotational speed range which lies outsidethe blocking ranges.